Preventing BI Emit material from lighting through objects
I tried to replicate, probably not identical, but it seems that indirect lighting is the factor mostly affecting that... I just reduced it from 1.0 to 0.2 here:



turning it off removes the colored shade completely (but also on the top face of the bottom cube):



Since indirect lighting here works only with "approximate" gather (if you use "raytrace" gather it works better and is very similar to turning off indirect like above), I guess that "approximate" word means: faster, but it does not fit well in all situations...

eg: